Article XL A PRELIMINARY STUDY OF THE NORTH 



AMERICAN OPOSSUMS OF THE GENUS 



DIDELPHIS. 



By J. A. ALLEN. 

 PLATES XXII-XXV. 



CONTENTS. 



Introduction ............ 149 



Skull and Dentition . . . . . . . . . .152 



Sexual Variation . . . . . . . . . .153 



Variation due to Age, etc. . . . . . . . .155 



Nasal Bones ........... 157 



Supernumerary Teeth, and other Abnormalities . . . .158 



Change in Color of the Ears with Age . . . . . . .159 



Nomenclature . . . . . . . . . . . .159 



Synopsis of Species and Subspecies . . . . . . . .160 



Postscript ............ 178 



Tables of Measurements and Ratios ....... 180 



INTRODUCTION. 



The present paper relates primarily to the forms of the genus 

 Didelphis occurring in Mexico and the United States, to which 

 area my material is mainly limited. It embraces, however, a few 

 specimens from Guatemala, Nicaragua, and Costa Rica, and many 

 examples from the Cauca region of western Colombia, the Santa 

 Marta region of eastern Colombia, and the island of Trinidad. 

 For nearly all of the Mexican material, which forms the principal 

 part of the collection, and much of that from the United States, 

 I am indebted to the kindness of Dr. C. Hart Merriam, Chief of 

 the Biological Survey of the United States Department of Agri- 

 culture, without the generous offer of which this paper would not 

 have been undertaken. The Mexican specimens were nearly all! 

 taken by Messrs. E. W. Nelson and E. A. Goldman, so well 

 known for their explorations in Mexico and Guatemala. I am 

 also indebted to Mr. G. S. Miller, Jr., and the authorities of the 

 National Museum for access to series of specimens from Yuca- 

 tan, Guatemala, and Costa Rica. 



The specimens examined number about 350 skins and nearly 

 100 additional skulls, of which 115 skins and 40 additional skulls 



149 



